Single commercial demonstrator marine current turbines are being installed at an increasing rate as a precursor to arrays or farms. Whilst initial arrays might only consist of a single row of devices aligned perpendicular to the predominant flow direction we can still expect device interaction effects as a function of spatial separation. As arrays get larger multiple rows will be required and this will require careful design to maximize energy yields and minimize device loads. Some knowledge can be taken from the wind energy industry approaches to farm designs whilst numerical and scale model testing provides, at present, the most promising indicators of potential interaction effects in marine current arrays. This paper combines data from literature and results obtained from scale model testing to help inform and shape early designs of marine current turbine arrays. It also includes analysis based on experimental data demonstrating positive interaction effects where fluid flows can be constrained to augment array performance.